1. Field of the Invention
The present invention relates to a voltage providing circuit for an electronic component of a computer, and particularly to a voltage providing circuit which can prevent an electronic component from being damaged when the computer is restarted because of a current failure.
2. General Background
Electronic components of a computer such as a memory, a central processing unit (CPU), a south bridge chip, and a north bridge chip need power to work. The power is provided by a motherboard of the computer.
Referring to FIG. 2, a conventional voltage providing circuit for a memory of a computer includes three metal-oxide-semiconductor field-effect transistors (MOSFETS) 10, 20, 30, a transistor Q10, and a controlled rectifier Q20. The MOSFET 10 is a P-channel-deplete MOSFET. The MOSFET 20 and the MOSFET 30 are N-channel-deplete MOSFETS. A control signal S10 is input to a base of the transistor Q10, a voltage V10 is input to a collector of the transistor Q10, and an emitter of the transistor Q11 is grounded. The voltage V10 is input to a source of the MOSFET 10, a gate of the MOSFET 10 is connected to the collector of the transistor Q10. A gate of the MOSFET 20 is connected to the collector of the transistor Q10, a drain of the MOSFET 20 is coupled to a drain of the MOSFET 10, and a source of the MOSFET 20 is grounded. A voltage V20 is input to a drain of the MOSFET 30, a gate of the MOSFET 30 is connected to the drain of the MOSFET 20, and a source of the MOSFET 30 is connected to a terminal Vout′ and is grounded via resistors R10 and R20. A cathode of the controlled rectifier Q20 is coupled to the drain of the MOSFET 10, a reference of the controlled rectifier Q20 is connected to a node between the resistor R10 and the resistor R20, and an anode of the controlled rectifier Q20 is grounded.
When the computer works normally, the control signal S10 is at a high level. So the transistor Q10 is turned on, and a voltage of the collector of the transistor Q10 is a low level voltage. Thus, the low level voltage is input to the gates of the MOSFET 10 and the MOSFET 20. So the MOSFET 10 is turned on and the MOSFET 20 is turned off. At the same time, the MOSFET 30 and the controlled rectifier Q20 are turned on. A voltage is output from the terminal Vout′ and is input to the memory. The voltage from the terminal Vout′ is: Vout′=Vref (voltage of the reference of the controlled rectifier)×[1+(R20/R10)]. In addition, a signal ‘power-ok’ (PWR-OK) is input to the memory. The memory will start to work when the PWR-OK signal is at a high level. If the computer is turned off improperly, the PWR-OK signal becomes low level immediately, but the control signal S10 becomes low level slowly. If the computer is restarted immediately, the PWR-OK signal is at a low level but the control signal S10 is still at a high level. So the voltage from the terminal Vout′ is input to the memory, but the memory has not started to work yet. Thus, the memory may be destroyed by the voltage from the terminal Vout′.
What is needed is a voltage providing circuit which can prevent an electronic component from being damaged when the computer is restarted.